The present invention relates to an oscillator having a variable frequency capable of varying an oscillating frequency.
A voltage controlled oscillator as shown in FIG. 1 has been used as the variable frequency oscillator. As shown, control voltage terminals 10 and 12 are respectively connected to the bases of NPN transistors 14 and 16. The transistors 14 and 16, of which the emitters are connected together, make up a differential circuit. The emitters of the transistors 14 and 16 are both connected to a constant current source 18.
The collector of the transistor 14 is connected to the base of an NPN transistor 20 and to a bias terminal 24 by way of an LC parallel resonance circuit 22. The collectors of the transistors 16 and 20 are connected to the bias terminal 24. The emitter of the transistor 20 is connected to an output terminal 26 and a bias current source 28, and to the emitters of the transistors 14 and 16 via a capacitor 30.
In the prior oscillator, a combination of the LC resonance circuit 22 and the capacitor 30 provides a phase shift or 2n.pi.(n: positive integer or 0) in a feedback loop, whereby allowing an oscillating signal to appear at the output terminal 26. Currents flowing through the differential pair of transistors 14 and 16 can be changed by changing a voltage applied between the control voltage terminals 10 and 12. The result is that the input impedance of the transistor 14 changes, a time constant given by the input impedance of the transistor 14 and the capacitance of the capacitor 30 changes, and consequently the oscillating frequency changes.
Since in the prior art the base voltages of the differential pair of transistors are controlled, the current flowing into one of the differential pair of transistors becomes zero if the voltage applied between the voltage terminals 10 and 12 reaches about 100 mV. It follows that the currents flowing in the differential pair of transistors are rendered constant if the voltage mentioned above exceeds 100 mV. Thus, a variable range of the control voltage is narrow which makes it difficult to set the oscillating frequency. The characteristic of an input impedance against the base voltage of the transistor is exponential. For this reason, a relationship of the control voltage between oscillating frequency has a poor linearity. In this respect, difficulty is again encountered in controlling the oscillating frequency. For operating the oscillator at a low frequency, the capacitance of the phase shifting capacitor 30 must be large. This must be avoided in fabricating the oscillator by integrated circuit technology.